Indium is supplied in the form of alloy targets with zinc and tin and paste as a material for transparent conductive films such as a semiconductor device and a touch panel and is widely used.
Ores incidentally contain indium in a very slight amount and also indium is unevenly distributed. So it is not easy to ensure a stable supply of indium. In addition, ores containing indium contain lead and zinc in a grossly excessive amount as compared to the amount of indium in many cases. Thus, when recovering indium there is a great problem with efficiently separating indium from lead and zinc.
Therefore, indium is obtained from ores, and furthermore indium is also obtained by recovering electronic circuit boards discarded in the city, defective items produced in production processes and, further, target materials which have been effectively finished and the like and recycling these. When the discarded electronic circuit boards and the like are recycled to obtain indium, however, there is still a problem with efficiently separating indium from gallium because indium and many types of valuable metal including gallium coexist in the discarded electronic circuit boards and the like.
As a method for extracting indium, a solution obtained by a two-stage neutralization treatment during the zinc leaching residue treatment step of zinc smelting is used as a solution to be extracted containing indium and gallium, and this solution to be extracted is adjusted to pH 2.4 to 3.6. Separately, a chelating agent is added to an organic solvent, and an extraction organic solvent having an organic phase in which a part of the protons in this chelating agent is substituted by an alkaline earth metal is also prepared. It is proposed that both these solutions be mixed to extract and isolate indium and gallium in the above solution to be extracted (see Patent Document 1). The method described in Patent Document 1, however, cannot be directly applied to a solution leached using an acid, for example, a low pH solution with a pH less than 2, and costs for a neutralizer to adjust pH, neutralization equipment, and much time for treatment are required.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2000-212658